An electro-acoustic impedance meter is an audiological instrument used to determine the performance of the middle ear system. The flexibility, or compliance, of the tympanic membrane and the mobility of the middle ear ossicular chain are measured as a combined system under various conditions or ear canal pressurization or evacuation and the compliance vs pressure data, thus obtained, may be plotted as the tympanogram for the tested ear. I subsequently refer to this combined complex compliance as ear canal compliance or simply compliance. The compliance measurement is based on the equivalent acoustical compliance of a hard-walled cavity and the usual unit of measurement is the equivalent volume expressed in cubic centimeters. Tympanometry is performed by inserting and sealing a pressure source, a sound source, and sound detector in the ear canal and evaluating the ear canal compliance as the ear canal pressure is adjusted. At a pressure of approximately +250mm H.sub.2 O, the tympanic membrane becomes essentially inelastic and the compliance measured at this pressure is termed the static, or base, compliance. As the ear canal pressure is decreased, a point of maximum, or peak, compliance occurs when the ear canal pressure is equivalent to the pressure existing in the subject's middle ear system. This pressure is termed the middle ear pressure. Further decrease in ear canal pressure again causes the tympanic membrane to become inelastic. The middle ear compliance is determined by subtracting the base compliance from the peak compliance. Middle ear compliance, middle ear pressure and the interrelationship between compliance and pressure are all of diagnostic significance.
If, at the point of peak compliance, the non-test, or contra-lateral ear is stimulated by a relatively loud sound, a contraction occurs in the muscles supporting the ossicular system, causing a partial stiffening of the tympanic membrane. This reaction is detected as a decrease in the measured compliance in the presence of the contra-lateral stimulus and is termed the acoustic reflex. The acoustic reflex is non-subjective and has diagnostic significance for both hearing and neurological problems.
My Automatic Electro-Acoustic Impedance Meter and other manually-operated electro-acoustic impedance meters seem to be misnamed, since both it and they operate by measuring the compliance, or equivalent volume, of the ear canal and tympanic membrane system. However, this terminology is in accordance with current American Speech and Hearing Association (ASHA) nomenclature and medical practice in the field, and stems from much earlier work done by purely acoustic methods.
Prior instruments in the impedance audiometry field are properly called impedance bridges, since they read the compliance magnitude by a complex manual bridge-balance technique, in which the balance control must be adjusted at each pressure setting and the compliance value read from a calibrated scale. These are cumbersome and slow to operate and due to the nature of the scales used, are difficult to read to a high precision. Prior instruments use a "3-tube" ear probe, employing individual flexible tubes for air pressure, sound source and sound detector, making the ear probe heavy and bulky. Prior instruments, employing the "3-tube" probe, exhibit significant acoustic coupling between the ear canal and the pressure pump system, thereby requiring electrical cancellation of a residual volume component in order to measure the relatively small, though significant, static compliance and middle ear compliance components. Prior instruments require manual balancing of the bridge system at the peak compliance point immediately before applying the contra-lateral stimulus in the measurement of the acoustic reflex. Prior instruments, due to the relatively broad frequency response to their sound detector amplifiers, respond to pulse beats, respiration, head movement and other unrelated influences.
It is therefore an object of my invention to provide an electro-acoustic impedance meter which is automatically maintained in balance by inverse feedback control.
It is another object of my invention to provide an electro-acoustic impedance meter which will display the results of its measurement in direct digital form, without reference to scales or the like.
It is another object of my invention to provide an electro-acoustic impedance meter employing capillary acoustic resistance means of isolation between the pressure system and the compliance measuring systems and thereby permitting the use of a lighter, smaller ear probe.
It is a further object of my invention to provide an electro-acoustic impedance meter which will display the presence of the acoustic reflex without prior balancing or other adjustment.
It is an additional object of my invention to provide an electro-acoustic impedance meter which includes narrow-band filter means to minimize the influences of head movement, pulse, breathing, etc. in the determination of compliance.
It is still another object of my invention to provide an electro-acoustic impedance meter which includes a wide range attenuator and tone source thereby permitting it to be used as a screening audiometer.
It is a general object of my invention to provide an electro-acoustic impedance meter which is simple and convenient to operate and is also reliable and rugged in construction to permit portable use.